This step is optional, feel free to skip to step 2 if you don't want to brown the butter! Add butter to a saucepan over medium-low heat. Whisk constantly until small brown flecks begin to appear in the butter and some bubbles form. The butter will release a nice nutty smell. Once it is browned, immediately remove it from the heat and place it in a separate bowl, as it will continue to cook a little bit off the heat.
Add the butter to a large bowl, and allow the butter to cool off for about 5 minutes before adding the sugars. Add brown and white sugar, and cream together with a hand mixer (or stand mixer, or whisk!) on medium-speed for 1-2 minutes until combined. Next, add in the vanilla extract and egg, and mix until combined and smooth.
Now, sift in the dry ingredients (flour, baking soda, salt, and cinnamon). Mix until just combined. Add in the muesli and chocolate chips, then mix until just combined. Now, cover with plastic wrap and place in the fridge to chill for 1 hour.
Preheat the oven to 375°F and line a baking sheet with parchment paper. Roll the cookies using a large cookie scoop into roughly 12 equal-sized balls. Top with more chocolate chips if desired, and place them on a parchment-lined baking sheet.
Place them on the center rack, and bake for 12-14 minutes, until the edges are set, but the inside is still a little bit gooey. Leave them on the baking sheet for a few moments so they can firm up. Transfer the cookies to a wire rack to cool completely, and sprinkle with flaky sea salt if desired.
Now, simply dig in and enjoy! Store leftovers in an airtight container on the counter for 4-5 days. After that, they will need to be transferred to the fridge or freezer.